chore(deps): update ubuntu docker tag to v20.10 (googleapis#130)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| ubuntu | final | minor | `20.04` -> `20.10` |

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.kokoro/docker/docs/Dockerfile

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from ubuntu:20.04
+from ubuntu:20.10
 
 ENV DEBIAN_FRONTEND noninteractive